1. Field of the Invention
This invention pertains to high pressure sodium vapor lamps. More particularly, the invention relates to a mercury-free high pressure sodium vapor lamp, which is dosed with sodium, xenon and elemental zinc to prevent an undesirable low-voltage operating mode at end-of-life.
2. Discussion of the Art
Traditional arc-discharge high pressure sodium (xe2x80x9cHPSxe2x80x9d) vapor lamps are described in U.S. Pat. No. 3,248,590 to Schmidt, entitled xe2x80x9cHigh Pressure Sodium Vapor Lamp.xe2x80x9d These lamps utilize a slender, tubular envelope of light-transmissive refractory oxide material resistant to sodium at high temperatures, suitably high-density polycrystalline alumina or synthetic sapphire. The filling has traditionally comprised sodium along with a rare gas such as xenon to facilitate starting and mercury for improved efficiency. The ends of the alumina tube are sealed by suitable closure members affording connection to thermionic electrodes which may comprise a refractory metal structure activated by electron emissive material. The ceramic arc tube is generally supported within an outer vitreous envelope or jacket provided at one end with the usual screw base. The electrodes of the arc tube are connected to the terminals of the base, that is to shell and center contact, and the interenvelope space is usually evacuated in order to conserve heat.
New environmental standards have necessitated that mercury be eliminated from the traditional arc-discharge HPS lamp design. These new designs are dosed only with sodium and xenon. Accordingly, as sodium is xe2x80x9clostxe2x80x9d by chemical reactions or diffusion, the voltage decreases markedly. The resultant low voltage mode is characteristic of a xenon discharge. Low voltage operation at end-of-life is very undesirable, resulting in an overheated ballast, which gives rise to reduced ballast life.
There is a particular need for a mercury-free high pressure sodium lamp, which maintains lamp voltage within established operating limits thereby ensuring that the lamp does not cycle (from high voltage) and the ballast is not overheated (from low voltage) at the end-of-life.
Briefly, in accordance with one embodiment of the present invention, a new and improved mercury-free high pressure sodium lamp is provided. The lamp is designed to prevent an undesirable low-voltage operating mode of the sodium-xenon discharge, which otherwise occurs when sodium is no longer available to participate in the arc discharge. The end-of-life operating voltage designed into the mercury-free HPS lamp is configured to be within a range acceptable to the ballast in accordance with established ANSI/IEC standards.
A principal advantage of the present invention is that an undesirable low-voltage operating mode of a sodium-xenon discharge associated with a mercury-free HPS lamp is prevented.
Another advantage of the present invention is that the end-of-life operating voltage for a mercury-free HPS lamp falls within a range acceptable to established ANSI/IEC standards.
Still another advantage of the present invention is that mercury-free HPS lamps can be produced in a normal product line without significant equipment changes or increase in lamp variable cost.
Still a further advantage of the present invention is that the mercury-free HPS lamps are direct replacements to standard HPS lamps, saving time and money in retrofit applications.
Still another advantage of the present invention is that mercury, a toxic substance according to the United States EPA""s TCLP guidelines, is eliminated from the HPS lamp.